Designing a user interface is something most people who use devices in general and computers specifically don't usually consider, so before we look at what I'm hoping to show, a quick word about how user interface impacts your life.
Think for a second about driving your car on a familiar road. Directly ahead of you a parked car begins to move into traffic! Without needing to think about it, you slam your hand on the center of the steering wheel to honk the horn and alert the driver.
So let's think about honking the horn. I don't know of a car sold in this country today that doesn't have its horn on the steering wheel, and I'll bet you don't either, because that's where the horn is, right? You know the interface of your car very well, and it probably feels natural, normal, and as it should be to find the horn right there.
Good thing you weren't driving a European car. Many European cars have the horn on the turn signal stalk! I'm not suggesting that this represents good or bad user interface design, I'm saying that interface design can become almost invisible to the end user. When you rent a car in another city in the US you have a very reasonable expectation that you'll rent a car that has its horn right where you expect it, this isn't information you'll need to re-learn.
So now we come to the iPad. Take a look at this video of a 2.5 year old girl as she first encounters an iPad. Granted, she's played with an iPhone before, but you're in that rare moment where you can see an interface being learned, absorbed and interacted with for the first time.
Did you see her figure out how to scroll from screen to screen? Watch her self-correct when she doesn't get an expected reaction from the device. Note how she expects the device to re-orient itself as she turns it. She's never seen the button to upscale an iPhone app for use on the iPad, but her reaction to finding it is instant acceptance of the concept. She shifts quickly from using/learning/adapting to the interface right into using the applications, so quickly in fact that we can almost see the moment when the interface falls away for her and just disappears into the background of her using the iPad.
She's learned the iPad interface in the same way that you've learned the horn location; it's not something you would talk about, it's simply obvious and organic.
I've argued with my friends since the iPad announcement that this device would change the way people work with computers. Sure, the iPad is a computer, but really the iPad is the first computer that isn't a computer; instead, it's all those things you use a computer for.